‘Magnetic transfer method’ refers to any activity related to particle movements produced by magnetism, such as e.g. sorting, collection, transfer, mixing or dosing of particles in the same liquid or from one liquid to another.
‘Particles’, ‘micro-particles’ or ‘magnetic particles’ refer to any small particles having a diameter mainly in the micrometer range, which can be moved by magnetism. Many different kinds of particles movable by magnetism are known, and the applications using them also vary widely. For example, the size of the particles used in microbiology is generally 0.01-100 μm, usually 0.05-10 μm. Known particles of this type include e.g. particles containing ferromagnetic, paramagnetic or supramagnetic material. Particles may also be magnetic in themselves, in which case they can be moved by means of any ferromagnetic body.
A device designed for the manipulation of micro-particles comprises an element which utilizes magnetism, and which is hereinafter referred to as a magnet. It may be a permanent magnet or an electromagnet which attracts ferromagnetic particles, or a ferromagnetic body which in itself is not magnetic but which still attracts magnetic particles.
The magnet is usually and preferably a round bar magnet. It may also be a bar of some other shape. However, the magnet need not be a bar at all. It may also be a short and wide body or a body of any shape. The magnet may also be composed of several bodies, such as magnets or ferromagnetic bodies.
The magnet has to be covered with a protecting element which protects the magnet from various adverse conditions and enables manipulation of micro-particles, such as binding and releasing. The structure of the protecting element may vary greatly, because it may consist of e.g. a thin film of elastic or stretchable material or even a cup made of hard plastic.
In general, micro-particles are used in solid phase to bind various biomolecules, cell organelles, bacteria or cells. It is also possible to immobilize enzymes on the surface of micro-particles, allowing effective utilization and further use of the enzymes. Most of so-called magnetic nanoparticles (<50 nm) can not be handled by means of ordinary permanent magnets or electromagnets, but they require the use of a particularly high magnetic gradient, as described in specification EP 0842704 (Miltenyi Biotec). Using ordinary permanent or electric magnets, it is usually possible to handle magnetic particles, such as micro-particles having a diametric size of about 0.1 μm or more. The viscosity of the sample may also be a significant factor making it difficult to collect the particles. The particles to be collected may originally have been suspended in a large quantity of liquid, from which a substance to be studied or even cells are desired to be bound to the surface of the particles. It is specifically important to be able to use large initial volumes in applications where components few in number are to be isolated for analysis. For example, efficient concentration of pathogenic bacteria from a large sample volume into a small volume is critical because it has a direct effect on the assay sensitivity and analysis time. At present, there is no sufficiently effective method for accomplishing concentration from a large volume to a small volume by using micro-particles. It would be advantageous to have a process of the above-described type as simple and efficient as possible.